The present inventors have become aware that changes in that part of the fetal electrocardiogram (FECG) known as the P-R interval (that is the time interval between the peak and the P wave and the R peak--see FIG. 3) herald a deterioration in the acid-base status of fetal blood. Previous reports of P-R interval changes have been both infrequent and, in part, contradictory. In papers, dated 1974 and before, and probably based on the analysis of recorded measurements taken during labor, one reported direct correlation between the duration of P-R interval and fetal heart rate (FHR) (although not when there was fetal tachycardia) and another that the P-R interval shortened with decelerating FHR. In another such paper a shortening of the P-R interval with acidosis was noted, while a different paper reported a long-term trend for the P-R interval to diminish towards the end of labor. A shortening or lengthening of the P-R interval with decelerations in FHR late in labor was also separately reported.